Maturation of thermoregulation in Zucker rats.

We compared the development of thermoregulatory abilities of fatty (fa/fa) and lean (Fa/-) Zucker rats as they grew from two weeks to eight months of age. The rectal temperature of lightly restrained littermates was continuously recorded for 2-6 h, and at ages before body weights of fatty and lean pups became significantly different, oxygen consumption was simultaneously measured. At the rearing temperature of 25 degrees C, hypothermia with a 30 percent lower oxygen consumption was apparent in fatty pups at 16 but not at 24 days. At 5 degrees C on day 25 fatty pups became hypothermic with a metabolic rate only 10-15 percent lower than lean pups. Three-month old fatty and lean rats defended the same core temperature not only at 25 degrees C, but even during 6 h without food at 5 degrees C. The core temperature of 5-8 month old fatty rats fell slightly but significantly below that of lean rats only during the sixth hour at 25 degrees C, but not during 6 h at 5 degrees C. If the capacity for thermogenesis in brown adipose tissue of fatty rats is less than in lean rats, its decreased contribution to cold defense must be completely made up in 25 degrees C-acclimated adults by more insulation and/or shivering.